1. Field of the Invention
The present invention relates to a thin film capacitor and, more particularly, a thin film capacitor formed on a semiconductor substrate.
2. Description of the Prior Art
Conventionally, as this type of thin film capacitor, a construction in which a dielectric film and an upper electrode are formed in order directly on an electrode area of a silicon substrate by a sputtering method has been disclosed on the Patent Application Laid-Open No. Hei 3-80562, hereafter referred to as Prior Art 1. Another construction in which a dielectric film and an upper electrode are formed in order on a conductive film, which comprises a first layer made of at least a type of high melting point metal such as, for example, Ta or Ti and a second layer made of at least a type of metal such as, for example, Pt or Pd on an electrode area of a silicon substrate has been disclosed on the Patent Application Laid-Open No. Hei 3-101260, hereafter referred to Prior Art 2. A further construction in which Pt or Pd is deposited as a lower electrode and a dielectric film and an upper electrode are formed in order on an insulative substrate made of sapphire or the like has been disclosed on IBM Journal of Res. Development, 68 (1969), p. 686.about.695, hereafter referred to as Prior Art 3.
As regards the thin film capacitors with the conventional constructions as described above, however, Prior Art 3 is disadvantageous in that, if the thin film capacitor is heat-treated after it has been made by forming a lower electrode, a dielectric film and an upper electrode in order on an insulative substrate, a reaction takes place between upper and lower electrodes and the dielectric film and consequently a leak current will increase. Prior Art 1 is disadvantageous in that, since the dielectric film is formed directly on the silicon electrode area by a sputtering method, the surface of the silicon electrode area is oxidized during formation of the dielectric film and the capacity of the thin film capacitor as a whole is limited due to silicon dioxide with a low dielectric constant which is produced on interfacial surfaces of the silicon electrode area and the dielectric film even though a dielectric film with a high dielectric constant is formed, whereas Prior Art 1 is advantageous in that the leak current is prevented from being substatially large owing to presence of the silicon dioxide layer in heat treatment after the thin film capacitor has been formed.
In case of a construction in which the first and second conductive films are formed on the silicon electrode area and the dielectric film and the upper electrode are formed thereon in order as in Prior Art 2, the capacity of the whole thin film capacitor can be enhanced while preventing formation of the layer with a low dielectric constant by an appropriate heat treatment but a leak current flowing through the dielectric film cannot be controlled, and particularly, in the case that the thickness of the dielectric film is 50 nm or less, a tunneling of the carrier is increased to enlarge the leak current. In addition, the construction in this case is also disadvantageous in that heat treatment after formation of the upper electrode will cause a reaction of the upper electrode and the dielectric film to result in an increase of leak current.
Accordingly, it is an object of the present invention which was made to solve such conventional problems to provide a thin film capacitor capable of preventing formation of a low dielectric constant layer and increasing of the leak current even after heat treatment by forming the lower and upper electrodes with at least two conductive films as the leak current characteristics may not be adversely affected due to a reaction of the electrode materials and the dielectric film.